Exceedance Function Help |
The exceedance function is a graph of the river stage (on the vertical axis)
versus the exceedance probability (on the horizontal axis).
Suppose that some stage, say 10 ft on day 1, is of your concern. You can read the corresponding exceedance probability 0.75 off the graph for day 1 and attach to it the following interpretation. Given all information available at the time of forecast preparation, the probability is 0.75 that at 7:00 am on day 1 the stage will be above 10 ft.
Probabilities of events other than the exceedance event can be determined as well. For example, the probability of the stage being below 13 ft is 1 - 0.14 = 0.86. The probability of the stage being between 10 ft and 13 ft is 0.75 - 0.14 = 0.61.
Symbols pinpoint five exceedance fractiles of the river stage - these are estimates that will be exceeded with probabilities 0.95, 0.75, 0.50, 0.25, 0.05. For example, the 0.75 exceedance fractile is 10 ft (lower dot); in other words, the probability is 0.75 that at 7:00 am on day 1 the stage will be above 10 ft. The median is 11 ft (circle); in other words, it is equally likely that at 7:00 am on day 1 the stage will be either below or above 11 ft.
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